Oracle has fixed a vulnerability in Java that a Russian cyberespionage group used to launch stealthy attacks earlier this year.

At the same time, Oracle fixed 153 other security flaws in Java and a wide range of its other products, it said Tuesday.

The Java vulnerability can be used to bypass the user confirmation requirement before a Web-based Java application is executed by the Java browser plug-in. This type of protection mechanism is commonly referred to as click-to-play.

The flaw was reported to Oracle by security researchers from Trend Micro, who first spotted the vulnerability in July in attacks launched by a Russian hacker group dubbed Pawn Storm that commonly targets military and governmental institutions from NATO member countries.

The vulnerability, tracked as CVE-2015-4902, was being used by the Pawn Storm attackers to enable the execution of a malicious Java application without user interaction. That application was designed to exploit a separate vulnerability that was also unpatched at the time, in order to install malware on computers.

Oracle patched the more serious code execution flaw (CVE-2015-2590) in July, but left the fix for the click-to-play bypass for the October quarterly security update released yesterday.

The flaw can't do much damage on its own, but in combination with code execution vulnerabilities it enables stealthy drive-by download attacks where users' computers can be compromised by simply clicking on a malicious URL. That's why it's very important to update Java as soon as possible to the latest available version.

"This case also highlights the importance of ensuring that when new security features (such as click-to-play) are introduced to a complex system like Java, it is a must to audit the communications of existing components with the new features," the Trend Micro researchers said in a blog post that explains in detail how an existing Java feature helped the security bypass.

In addition to addressing this vulnerability, the new Java update patches 24 other security flaws, most of which can be exploited remotely without authentication.